Add user_id for sample volume.snapshot.size

Volume Snapshot details include user_id since version 3.41
https://developer.openstack.org/api-ref/block-storage/v3/#list-snapshots-and-details

Change-Id: I13ab9733d79a19bc191792ee8c7d11ab477df52e

ceilometer/tests/unit/volume/test_cinder.py

@@ -65,6 +65,7 @@ SNAPSHOT_LIST = [
           u'os-extended-snapshot-attributes:project_id':
               u'6824974c08974d4db864bbaa6bc08303',
           u'size': 1,
+          u'user_id': u'be255bd31eb944578000fc762fde6dcf',
           u'updated_at': u'2016-10-19T07:56:55.000000',
           u'id': u'b1ea6783-f952-491e-a4ed-23a6a562e1cf',
           u'volume_id': u'6f27bc42-c834-49ea-ae75-8d1073b37806',
@@ -135,6 +136,8 @@ class TestVolumeSnapshotSizePollster(base.BaseTestCase):
         self.assertEqual('volume.snapshot.size',
                          volume_snapshot_size_samples[0].name)
         self.assertEqual(1, volume_snapshot_size_samples[0].volume)
+        self.assertEqual('be255bd31eb944578000fc762fde6dcf',
+                         volume_snapshot_size_samples[0].user_id)
         self.assertEqual('6824974c08974d4db864bbaa6bc08303',
                          volume_snapshot_size_samples[0].project_id)
         self.assertEqual('b1ea6783-f952-491e-a4ed-23a6a562e1cf',

ceilometer/volume/cinder.py

@@ -73,7 +73,7 @@ class VolumeSnapshotSize(_Base):
                 type=sample.TYPE_GAUGE,
                 unit='GB',
                 volume=snapshot.size,
-                user_id=None,
+                user_id=snapshot.user_id,
                 project_id=getattr(
                     snapshot,
                     'os-extended-snapshot-attributes:project_id'),